The subject matter herein relates generally to electrical cables that provide shielding around signal conductors.
Shielded electrical cables are used in high-speed data transmission applications in which electromagnetic interference (EMI) and/or radio frequency interference (RFI) are concerns. Electrical signals routed through shielded cables may radiate less EMI/RFI emissions to the external environment than electrical signals routed through non-shielded cables. In addition, the electrical signals being transmitted through the shielded cables may be better protected against interference from environmental sources of EMI/RFI than signals through non-shielded cables.
Shielded electrical cables are typically provided with a shield layer formed by a metal foil. Signal conductors are typically surrounded by an insulation layer, and the metal foil is subsequently wrapped around the insulation layer to provide shielding for the signal conductors interior of the metal foil. For example, in some known applications a metal foil is spiral wrapped around the insulation layer, such that adjacent loops or revolutions of the metal foil at least partially overlap, which is referred to as overlay, to prevent EMI/RFI leakage across the shield layer. An adhesive polymeric tape, such as Mylar® (a polyester film manufactured by Dupont), may be wrapped around the outside of the metal foil to hold the wrapped metal foil in place.
Wrapping a metal foil as a shield layer in a shielded electrical cable has disadvantages. For example, helically wrapping the foil layer and the tape layer over the foil layer results in discontinuities that affect the signal integrity. The frequency or repetitiveness of the tape overlay causes geometrical changes within the signal pair construction. Tape overlay lengths over the signal conductors play a fundamental role in frequency bandwidth, such that it has a direct effect on attenuation or signal loss. For example, short overlay lengths generally push the attenuation to higher bandwidths, while longer overlay lengths push the attenuation to relatively lower bandwidths. Increasing the overlay may improve insertion loss by pushing the attenuation outside of an operational range of bandwidths, although it may also undesirably increase the rigidity or stiffness of the cable, as well as increase manufacturing time and material usage. Thus, there is a trade-off between signal integrity, flexibility, and manufacturing costs. Furthermore, in some cables, it may be desirable to electrically connect together the shield layers that surround different signal conductors. But, since the adhesive tape on the outside of the shield layer insulates the shield layer, a portion of the tape must be removed or penetrated, or a drain wire must be extracted through the tape layer, in order to access the shield layer.
A need remains for an electrical cable that improves signal performance and simplifies manufacturing.